The present invention relates to a contact image sensor having photoconductive detecting elements of amorphous silicon (a-Si) arranged in a linear array of approximately the same length as the object to be read.
A circuit for a conventional linear image sensor for matrix reading is shown in FIG. 2. The linear image sensor 10 is made up of photoconductive detecting elements 1 which are linearly arranged at a density of 8 elements, 12 elements, or 16 elements per millimeter. The total number of detecting elements in a linear image sensor for A-4 size paper is 1728 if it has 8 elements per millimeter. The 1728 elements are divided into 32 groups, each consisting of 54 elements. Positive electrodes of the 54 elements as a group are connected to first switches 20 in common. Negative electrodes at the corresponding positions in each group of the 54 elements are connected to second switches 30 in common. Therefore, the number of first switches 20 is 32 and the number of second switches 30 is 54. First switches 20 are connected to second switches 30 via a power source 40 (6-10 V) and a current detector 50. To achieve reading with this linear image sensor, the resistance of the individual elements 1 is measured consecutively by turning on and off second switches 30 sequentially, with one group of first switches 20 closed. If one of the detector elements 1 receives light, it decreases in resistance, permitting a current to flow through the detector 50. Without light, the detector element has a high resistance and prevents a current from flowing through the detector 50. Thus the detector elements 1 distinguish black parts from white parts on the original across which the linear image sensor 10 is passed.
A conventional linear image sensor based on amorphous silicon is constructed as shown in FIG. 3. There is shown a soda glass substrate 2 which is coated with an SiO.sub.2 film 21 to hold back sodium. On the substrate 2 are formed matrix metal electrodes 31, 32, 33, 34 of chromium. (Only four electrodes are shown.) They are covered with a silicon nitride (SiN) film 4. These matrix electrodes are connected to second switches 30 shown in FIG. 2. On the substrate 2 is also formed an intrinsic amorphous silicon (a-Si) film 51. The intrinsic a-Si film 51 is connected to a metal electrode 6 via an n-type a-Si film 52. The metal electrode 6 is connected to switches 20 shown in FIG. 2. The intrinsic a-Si film 51 is connected also to metal electrodes 61, 62, 63, 64 via an n-type a-Si film 52. The lead electrodes 61, 62, 63, 64 are connected to the matrix electrodes 31, 32, 33, 34, respectively, via the contact hole 41 formed in the SiN film 4. The array of the photosensors is covered with a transparent protective film 71 of SiN or epoxy resin, and the matrix circuit is covered with an opaque protective film 72.
The conventional image sensor as mentioned above has a weak point arising from the fact that the leader electrodes 61, 62, 63, 64 come into contact with the matrix electrodes 31, 32, 33, 34, respectively, at as many as 1728 places in the case of an image sensor for A-4 size paper. Thus, the image sensor becomes defective even if it has only one short circuit caused by a pinhole defect in the insulation film 4. For this reason, the defective fraction of conventional image sensors is invariably higher than 50%. It is thus an object of the present invention to solve the above-mentioned problem and to provide a linear image sensor at a low fraction defective.